1. Technical Field
The present invention relates to optical fiber communication and more particularly to systems and methods for downstream and upstream optical transmission with wavelength reuse using a centralized lightwave.
2. Description of the Related Art
Research and development has been focused on the next generation of passive optical network (PON) access networks, such as wavelength division multiplexing (WDM) passive optical networks (WDM-PONs) and time division multiplexing PONs (TDM-PONs). However, TDM-PON systems need complex scheduling algorithms and framing technology to support different applications. WDM-PON still lacks the flexibility to dynamically allocate the bandwidth among optical network units (ONUs).
Wavelength reuse is a cost-effective solution in WDM-PON systems, and several re-modulation schemes have been proposed. However, these modulation schemes have at least the following disadvantages: 1) color ONUs are needed, which means that ONUs have an inflexibly assigned wavelength; and 2) poor chromatic dispersion (CD) tolerance for high speed (e.g., 10 Gb/s) signal transmission.
Wavelength reuse in proposed WDM-PONs employs phase modulated signals. Here, downstream differential phase shift keyed (DPSK) signals are generated by a phase modulator (PM). After a transmission fiber, the downstream DPSK signal is launched into the ONU via an optical circulator (OC), and 10% of optical power is received by an optically pre-amplified receiver, which consists of an optical variable optical attenuator (VOA), erbium-doped fiber amplifier (EDFA), a delayed interferometer (DI) for DPSK demonstration and a PIN receiver. The remaining optical power is launched into another PM to rewrite the downstream signal for the upstream data. One electrical delay line and electrical buffers are needed to align between the downstream and the applied electrical signals to the PM. In this way, phase modulators and the DI are required in the system. However, this configuration greatly increases the cost and complexity of the system.